This invention relates to devices for obtaining information about radiation sources.
Visible light can be both reflected and refracted. The ordinary camera takes advantage of refraction by using an optical lens to refract and focus the visible light coming from an object, in order to produce an image of the object on film. However, electromagnetic radiation of higher frequency than vacuum ultraviolet (such as X rays and gamma rays, together referred to hereinafter as "gamma radiation") cannot be efficiently either reflected or refracted. Therefore the formation of an image of a source of gamma radiation has been achieved with the use of a collimator, which operates in somewhat similar fashion to the old pinhole camera. The pinhole camera permits the light from an object to pass in a straight line through a pinhole in the camera box, to produce an inverted image on the film.
Collimators with an array of parallel channels, such as the one depicted in FIG. 1, have been used in the imaging of gamma radiation sources. With the axes of the channels pointed toward a gamma radiation source, the channels are generally the same size in both dimensions normal to the axis; usually the channels are circular, triangular, or square in cross section. In FIG. 1, with the walls or septa of each channel made of lead to absorb gamma radiation, and with a radiation detector (not shown) placed on the side of the collimator opposite the source, the radiation that can pass from a point source through a particular channel and reach the detector is defined by the solid angle (A) subtended by the base of the collimating channel 2. Spatial resolution of such a collimator is improved by reducing the solid angle. However, the sensitivity of each channel, which increases as does the amount of radiation passing through the channel, is improved by increasing the solid angle. Of course it is desirable to improve both spatial resolution and sensitivity, the latter particularly so that the necessary time of observation may be reduced.
Turning to the radiation detector, it is known to use the photoconductor as the basic element of such a detector. However, in known arrays of photoconductor detector elements the photon absorption distance and the interelectrode distance correspond to the same photoconductor dimension, and thus are generally the same length. It would be desirable to make the photon absorption distance large with respect to the interelectrode distance, for the larger the absorption distance, the better the sensitivity, because a larger fraction of the incident photons are collected, and the smaller the interelectrode distance, the greater the efficiency, and also the rapidity, of collection at the electrodes of electrical signals created in the photoconductor body by incident photons.